Although envisioned since the late 1980s, hybrid PET/MR systems only became commercially available in the last few years and more than a decade later than hybrid PET/CT. This is explained by the technological challenges originating from the combination of these two very different imaging modalities. Manifold interferences between the two modalities (in terms of B0, Gradient, RF, Temperature, Photon Attenuation, Space Constraints, Workflow, …) needed to identified, understood and solved.
The combination of Positron Emission Tomography (PET) and Magnetic Resonance Imaging (MRI) forms a powerful new imaging modality PET/MR. The advantages of hybrid and concurrent PET/MR are manifold including superior anatomical, functional and molecular imaging, reduced radiation exposure and improved patient comfort and workflow (1–3).
Although envisioned since the late 1980s, hybrid PET/MR systems only became commercially available in the last few years (4–8) and more than a decade later than hybrid PET/CT (9). This is explained by the technological challenges originating from the combination of these two very different imaging modalities (10). Manifold interferences between the two modalities (in terms of B0, Gradient, RF, Temperature, Photon Attenuation, Space Constraints, Workflow, …) needed to identified, understood and solved. For example, this resulted in the development of new solid-state photon detectors (SSPMs), replacing conventional photo multiplier tubes (PMTs). In addition to hardware compatibility, also attenuation correction (required for quantitative PET imaging) needed to be addressed. While in PET/CT, attenuation information can be readily derived from CT images, MR images primarily contain proton density and relaxation (T1, T2) information unrelated for PET photon attenuation.
The simultaneous acquisition of PET and MR also offers opportunities in terms of MR-based PET motion correction, PET partial volume correction, Joint PET and MR image reconstruction, and MR-derived arterial input function estimation (as required for kinetic modeling of dynamic PET data).
The ability of PET/MR to simultaneously image anatomical, functional and molecular characteristics of a tumor, renders it an ideal choice for Radiation Therapy Planning (RTP) in terms of improved tumor delineation, assessing tumor response in follow-up investigations and biological individualization (aka dose painting) (11–15).
1. Pichler BJ, Kolb A, Nägele T, Schlemmer H-P. PET/MRI: paving the way for the next generation of clinical multimodality imaging applications. J. Nucl. Med. Off. Publ. Soc. Nucl. Med. 2010;51:333–336. doi: 10.2967/jnumed.109.061853.
2. Quick HH. Integrated PET/MR. J. Magn. Reson. Imaging JMRI 2014;39:243–258. doi: 10.1002/jmri.24523.
3. Wehrl HF, Sauter AW, Divine MR, Pichler BJ. Combined PET/MR: A Technology Becomes Mature. J. Nucl. Med. 2015;56:165–168.
4. Schlemmer H-PW, Pichler BJ, Schmand M, et al. Simultaneous MR/PET imaging of the human brain: feasibility study. Radiology 2008;248:1028–1035. doi: 10.1148/radiol.2483071927.
5. Zaidi H, Ojha N, Morich M, Griesmer J, Hu Z, Maniawski P, Ratib O, Izquierdo-Garcia D, Fayad ZA, Shao L. Design and performance evaluation of a whole-body Ingenuity TF PET-MRI system. Phys. Med. Biol. 2011;56:3091–3106. doi: 10.1088/0031-9155/56/10/013.
6. Delso G, Fürst S, Jakoby B, Ladebeck R, Ganter C, Nekolla SG, Schwaiger M, Ziegler SI. Performance measurements of the Siemens mMR integrated whole-body PET/MR scanner. J. Nucl. Med. 2011;52:1914–1922.
7. Veit-Haibach P, Kuhn FP, Wiesinger F, Delso G, von Schulthess G. PET–MR imaging using a tri-modality PET/CT–MR system with a dedicated shuttle in clinical routine. Magn. Reson. Mater. Phys. Biol. Med. 2013;26:25–35.
8. Levin CS, Maramraju SH, Khalighi MM, Deller TW, Delso G, Jansen F. Design Features and Mutual Compatibility Studies of the Time-of-Flight PET Capable GE SIGNA PET/MR System. IEEE Trans. Med. Imaging 2016:1–1. doi: 10.1109/TMI.2016.2537811.
9. Beyer T, Townsend DW, Brun T, Kinahan PE, Charron M, Roddy R, Jerin J, Young J, Byars L, Nutt R. A combined PET/CT scanner for clinical oncology. J. Nucl. Med. Off. Publ. Soc. Nucl. Med. 2000;41:1369–1379.
10. Dahlbom M ed. Physics of PET and SPECT imaging. Boca Raton, FL: CRC Press, Taylor & Francis Group; 2017.
11. Beavis AW, Gibbs P, Dealey RA, Whitton VJ. Radiotherapy treatment planning of brain tumours using MRI alone. Br. J. Radiol. 1998;71:544–548. doi: 10.1259/bjr.71.845.9691900.
12. Nyholm T, Jonsson J. Counterpoint: Opportunities and challenges of a magnetic resonance imaging-only radiotherapy work flow. Semin. Radiat. Oncol. 2014;24:175–180. doi: 10.1016/j.semradonc.2014.02.005.
13. Dirix P, Haustermans K, Vandecaveye V. The value of magnetic resonance imaging for radiotherapy planning. Semin. Radiat. Oncol. 2014;24:151–159. doi: 10.1016/j.semradonc.2014.02.003.
14. Schmidt MA, Payne GS. Radiotherapy planning using MRI. Phys. Med. Biol. 2015;60:R323–361. doi: 10.1088/0031-9155/60/22/R323.
15. McGee KP, Hu Y, Tryggestad E, Brinkmann D, Witte B, Welker K, Panda A, Haddock M, Bernstein MA. MRI in radiation oncology: Underserved needs. Magn. Reson. Med. 2016;75:11–14. doi: 10.1002/mrm.25826.